The present invention relates to a method and apparatus for recovering uranium and/or related values from an ore, either at the surface or in situ. More particularly, the present invention relates to a method and apparatus for protecting the ion-exchange resins in a uranium recovery operation from poisoning by certain oxidized sulfur compounds normally found in typical leach solutions.
In a typical uranium recovery operation, uranium-bearing ore (either mined or in situ) is contacted with a leach solution which oxidizes and dissolves the uranium values from the ore. The pregnant leach solution is then flowed through one or more columns containing a strong base, anionic, ion-exchange resin which adsorbs the uranium values from the leach solution. When the resin in a column is sufficiently loaded with uranium values, the flow of leach solution is switched to another column and a second solution or eluant is flowed through the loaded column to desorb the uranium values into the eluant, now called the eluate when loaded with uranium values. Due to the eluant composition, the concentration of uranium values in the eluate will be greater than was the concentration in the leach solution. The eluate is then processed to recover the uranium values.
However, in operations of this type, as the leach solution (e.g. a carbonate solution and an oxidant) passes through the ore, not only are the uranium values oxidized but, also, sulfur and/or sulfur compounds (e.g. elemental sulfur, pyrites, hydrogen sulfice, et al) in the ore are oxidized to both sulfate compounds and intermediate sulfur oxide compounds (e.g. polythionates, thiosulfates, and sulfites). These oxidized sulfur compounds will dissolve along with the uranium values into the leach solution and will be produced therewith to the surface.
When the leach solution is flowed through the ion-exchange resin to remove the uranium values, some of the oxidized sulfur compounds will also be adsorbed onto the resins. The normal elution of the resin to remove the uranium values will also remove most of these sulfur compounds. It is known, however, that polythionates are more strongly adsorbed on commercially available, anionic resins and will not be readily removed by standard elution. This chemical "poisoning" of the resin caused by polythionate adsorption will continue to build with each loading and elution cycle until the uranium loading capacity of the resin becomes too low in a commercial recovery operation.
In present commerial operations, when the resin becomes substantially poisoned with polythionates, it must be regenerated to remove the poison. This is now commonly accomplished by flushing the resin with a concentrated caustic solution. Not only is this regeneration procedure expensive and time consuming, but, more importantly, the use of the concentrated caustic solution leads to the destruction and/or defunctionalization of the resin thereby significantly reducing the operational life of the very expensive resin.
In view of the above, it is obvious that a need exists for a simpler and more efficient method to protect the resin from poisoning by the oxidized sulfur compounds normally found in the leach solution.